The present invention concerns a dishwasher, in particular of the domestic type, provided with at least one improved agitator to spray dishes supported in a suitable basket.
As is known, agitators of a dishwasher rotate due to the effect of reactions caused by water jets coming out of respective pressure nozzles. The pressure nozzles are normally fixed so as to emerge from the agitator, which then rotates in a predetermined direction. Obviously, the agitators also include nozzles that spray dishes, which can be fixed and/or rotating. In any case, the spray nozzles do not substantially effect the rotation of the agitator.
In some cases dishes, incorrectly positioned in the dishwasher, interfere with and block the agitator. This compromises the results of washing and the machine may be insufficiently cooled because of the thermal resistance of the water. To eliminate these inconveniences, dishwashers have been proposed in which there is an electromagnetic sensor capable of activating an alarm device if an agitator rotates below a predetermined speed, as described for example in EP 0,053,231.
A similar mechanical solution is described, for example, in DE 4,020,899, in which a valve device and a blocking device are capable of reducing the rotation of the agitator, permitting the washing program to proceed only if the agitator itself is not interfered with by the dishes during an initial "exploratory" rotation. If there is interference the functioning of the dishwasher is interrupted or an alarm device is activated.
Such solutions are unduly complicated and costly, and in any case they do not solve the functional problems arising from a possible blockage of the agitator. They are limited to signaling the blockage, so that the user can intervene appropriately to restore a condition of proper functioning.
A more effective solution that operates automatically is described in U.S. Pat. No. 3,797,509, in which the direction of rotation of the agitator in a dishwasher can be reversed by means of using a spherical bi-stable commutation valve that selectively feeds opposing pressure nozzles provided on the agitator. The alternating commutations of the spherical valve are controlled according to a preset program by means of suitable pauses in the functioning of the pump that feeds the agitator. Therefore, repeated reversals of the direction of rotation of the agitator require corresponding pauses of the pump and the corresponding drag motor, as well as a suitable programming system for the machine. As a result, the hydraulic noise caused by the intermittent functioning of the pump is unduly high. The drag motor is subject to repeated demands, which reduce its lifetime, and programming of the machine requires a relatively complex system. In any case, the spherical commutation valve has a critical function, being subject to sticking caused by contact with particles of dirt suspended in the water. The valve also unduly increases the vertical space needed by the agitator.